All good advice. Keep in mind that a topspin ground stroke will always be about creating a ratio of drive to spin. Just change that ratio according to how the ball comes to you. I generally drive low balls flatter and higher balls with topspin.

For a flat shot the trajectory of the ball has little to no arc all the way to when it bounces. For a shot that is flattened the trajectory of the ball also has little to no arc but as it goes down to bounce, the ball drops with a steeper angle because of the spin.

Racket technology has a lot to do with this. It allowed the strokes to evolve into the "modern" strokes we are seeing now.